One of the key components of any computer system is a place to store data. Computer systems have many different places where data can be stored. One common place for storing massive amounts of data in a computer system is on a disk drive. The most basic parts of a disk drive are a disk that is rotated, an actuator that moves a transducer to various locations over the disk, and electrical circuitry that is used to write and read data to and from the disk. The disk drive also includes circuitry for encoding data so that it can be successfully retrieved and written to the disk surface. A microprocessor controls most of the operations of the disk drive as well as passing the data back to the requesting computer and taking data from a requesting computer for storing to the disk.
The transducer is typically housed within a small ceramic block. The small ceramic block is passed over the disk in transducing relationship with the disk. The transducer can be used to read information representing data from the disk or write information representing data to the disk. When the disk is operating, the disk is usually spinning at a relatively high RPM. These days common rotational speeds are 5100 and 7200 RPM. Rotational speeds of 10,000 RPM and higher are contemplated for the future. At such speeds, the very small ceramic block flies on a very thin layer of gas or air. In operation, the distance between the small ceramic block and the disk is very small. Currently "fly" heights are about 0.0003 mm. In some disk drives, the ceramic block does not fly on a cushion of air but rather passes through a layer of lubricant on the disk.
Information representative of data is stored on the surface of the memory disk. Disk drive systems read and write information stored on tracks on memory disks. Transducers, in the form of read/write heads, located on both sides of the memory disk, read and write information on the memory disks when the transducers are accurately positioned over one of the designated tracks on the surface of the memory disk. The transducer is also said to be moved to a target track. As the memory disk spins and the read/write head is accurately positioned above a target track, the read/write head can store data onto a track by writing information representative of data onto the memory disk. Similarly, reading data on a memory disk is accomplished by positioning the read/write head above a target track and reading the stored material on the memory disk. To write on or read from different tracks, the read/write head is moved radially across the tracks to a selected target track. The data is divided or grouped together on the tracks. In some disk drives, the tracks are a multiplicity of concentric circular tracks. In other disk drives, a continuous spiral is one track on one side of a disk drive. Servo feedback information is used to accurately locate the transducer. The actuator assembly is moved to the required position and held very accurately during a read or write operation using the servo information.
The actuator assembly is composed of many parts that contribute to the performance required to accurately hold the read/write head in the proper position. There are two general types of actuator assemblies, a linear actuator and a rotary actuator. The rotary actuator includes a pivot assembly, an arm, a voice coil yoke assembly and a head gimbal suspension assembly. The rotary actuator assembly pivots or rotates to reposition the transducer. A suspension or load beam is part of the head gimbal suspension assembly. Currently the length of the arm is about equal to the length of the suspension. The length of the arm and the length of the suspension determine, in part, the resonance frequency of the actuator assembly.
One end of the suspension is attached to the actuator arm. The read/write head is found attached to the other end of the suspension. One end of the actuator arm is coupled to a pivot assembly. The pivot assembly is in turn connected to a servo motor system through the voice coil yoke. The other end is attached to the head gimbal suspension assembly. The head gimbal suspension assembly allows the read/write head to gimbal for pitch and roll to follow the topography of the imperfect memory disk surface. The head gimbal assembly also restricts motion with respect to the radial and circumferential directions of the memory disk. The suspension is coupled to the actuator arm as part of the mounting support holding the pivot support and coupled to the servo motor. Currently, the pivot assembly is mounted within an opening in a unitized E-block. The E-block includes arms for mounting the suspension on one end and a voice coil yoke on the other end. U.S. Pat. No. 5,283,704 issued to Reidenbach illustrates another actuator system composed of individual components instead of the unitized E-block. This actuator system is "built up" from an actuator arm, spacer rings, a separate voice coil yoke frame assembly, and a separate bearing cartridge. A voice coil is located on the voice coil yoke. The voice coil and magnets attached to the housing of the disk drive form a voice coil motor. The disk drive includes a feedback control loop to enable accurate positioning of the transducer. The disk drive system sends control signals to the voice coil motor to move the actuator arm and the suspension supporting the read/write head across the memory disk in a radial direction to the target track. The control signals indicate to the motor the magnitude and direction of the displacement. The control signals can also be used to maintain the position of the read/write head or transducer over a particular track.
Actuator arms act as spring-mass-damper systems and have resonant frequencies that can degrade the performance of the servo system. Every closed loop servo motor system has a predetermined bandwidth in which resonances occurring within the bandwidth degrade the performance of the servo motor system. The actuator arm is one key source of unwanted resonances. Accordingly, the bandwidths of most servo motor systems are designed such that resonances of the actuator arm occur outside the bandwidth. Each actuator arm has a unique resonance characteristic. Current actuator arms are made of stainless steel, aluminum or magnesium. Suspensions are typically made of stainless steel. The resonance characteristics of the arm are such that the bending modes and torsion modes have frequencies that are within the same frequency range as the suspension and the magnetic storage disk (1 kHz to 8 kHz). Great care must be used when designing an actuator system to prevent alignment of resonance modes that would create very high gains and an unstable servo performance.
In other words, in the presence of a resonance, the transducer or read/write head will vibrate causing it to pass across the desired track. When the resonances of the disk and suspension align or are about the same frequency, the frequency response is amplified so that the amplitude of the vibration is higher and the read/write head travels farther away from the desired track during the track crossings.
Stainless steel or metal arms could be made thicker to increase the bending and torsion mode frequencies, but the greater mass significantly degrades the performance of the actuator assembly by increasing the moment of inertia of the arm. Inertial increase will decrease the access time to transition between data tracks and increase the current requirements necessary to move the voice coil motor. Increased current results in increased heat within the disk enclosure and increased power requirements.
A thicker steel will also result in a higher mass assembly that will cause significant degradation of shock resistance of the disk drive system. Higher mass assemblies also imply less stability in the form of head lift-off. When a large shock impulse in the vertical direction is applied to the actuator arm, the head gimbal assembly "lifts off" and lands back on the disk surface. This damages the memory disk. Other metals such as aluminum have been used, but the key parameter determining the resonance characteristics of the actuator arm is the stiffness to mass ratio of the material, which is about the same for aluminum and stainless steel. Of currently available materials that have been used as actuator arms, only beryllium and ceramics have significantly higher stiffness to mass ratios over that of currently used stainless steel or aluminum. Beryllium is quite expensive and difficult to process while ceramics are prone to crack, particularly under shock load conditions. As a result, these materials have not become market acceptable.
The demand for greater track density is increasing steadily, so increasing the performance of the actuator assembly by increasing the resonance frequencies of the arm, the suspension and the entire actuator assembly is becoming a requirement for future systems. There is also a need for a disk drive system with lower access times. Furthermore, there is a need for a more stiff actuator arm since stiff arms have higher resonant frequencies associated with the various bending modes. There is also a need for an actuator arm where the stiffness can be controlled in various directions so that the resonances can be changed to frequencies other than the resonant frequencies of other components of the disk drive. There is still a further need for an arm that has a high stiffness to mass ratio such that the length of the arm can be extended to replace the suspension material, which typically has a lower stiffness to mass ratio. Suspensions are typically made out of stainless steel.